The present invention relates generally to measurement test instruments and more specifically to a measurement test instrument having a voltage management system for one or more accessory devices.
A typical measurement probe for acquiring electrical signals from a device under test has a probing head connected to a termination box via a transmission cable, such as a coaxial cable. The probing head has an electrically conductive hollow tube with a substrate disposed therein. The substrate has passive and/or active circuitry to prevent loading of the test device. One end of the hollow tube has an insulating plug disposed therein with a coaxially disposed probing tip or socket extending out of the plug in both directions. The portion of the probing tip or socket extending into the hollow body is electrically connected to the substrate. The other end of the substrate is coupled to one end of the transmission cable and to voltage power lines, clock lines and data lines, if present. The other end of the transmission cable is coupled to compensation circuitry in the termination box. The termination box has a coaxial signal connector, such as a BNC type connector, and power, clock and data connectors for connecting the measurement probe to a measurement test instrument, such as an oscilloscope or the like.
High frequency measurement probes require high input resistance and low input capacitance. Generally, these types of probes have an active FET input for low circuit loading and have a frequency bandwidth up to 4 Ghz or more. An example of such a probe is the P6245 Active FET Probe, manufactured and sold by Tektronix, Inc., Beaverton, Oregon and assignee of the instant application. The P6245 has a male BNC type signal connector surrounded by a series of voltage, clock and signal pins as described in U.S. Pat. No. 4,708,661. The voltage pins provide xc2x15 V and xc2x115 V to the probe head and termination box circuitry and a memory device disposed in the termination box. The memory device contains stored data related to the specific probe, such as probe type, correction factor values and offset values. A clock signal is provided to the memory device via the clock pin and the data stored in the memory is read out to the measurement instrument via the data pin.
A flexible or rigid printed circuit surrounds the mating female BNC connector on the front panel of the measurement test instrument. The printed circuit has a plurality of circuit paths ending in termination points. The pins of the measurement probe engage the termination points on the flexible printed circuit. The circuit paths are connected to voltage supplies, clock circuitry and the data line within the instrument and provide all of the signal and power lines to power and operate the active probe. When the measurement probe is connected to the measurement test instrument, power is immediately supplied to the probe.
The computer industry has developed techniques for adding and removing cards or peripheral devices from the powered computer system called xe2x80x9chot swappingxe2x80x9d. As the term implies, the connectors into which the card or peripheral device is plugged into have supply voltages on the voltage pins of the connectors. Various techniques are used to power-up the card or peripheral without generating an in-rush of current or loading the system power supplies. One technique uses variable length contacts on the card or peripheral to initially couple the voltage supplies on the connector to a RC ramping circuit and associated control circuitry. The RC circuit and associated circuitry ramps up the supply voltages on the card to a desired level before the voltages are supplied to the rest of the circuitry on the card. Another technique requires a user to power down a particular connector slot, insert the card or peripheral, and then power up the connector slot.
One drawback to the existing techniques of xe2x80x9chot swappingxe2x80x9d is that the card or peripheral is powered up before the system determines the type of card or peripheral it is and configures it for the system. The system does not know if the card or peripheral is compatible, if it draws an excessive amount of current, or if the added card or peripheral will cause the total amount of current drawn by all of the connected devices to exceed the maximum current rating for the one or more of the power supplies. In addition, most of the xe2x80x9chot swappingxe2x80x9d techniques require the RC network and switching circuitry to located in the card or peripheral device. Providing such circuitry in a measurement probe will increase the size of the termination box and add cost to the probe.
What is needed is a measurement test instrument voltage management system for accessory devices, such as a measurement probe, that verifies the connected device is a valid and supported device prior to applying power to the device. The voltage management system should also monitor each connected device to determine if a newly connected device results in excessive current draw from one or more power supplies. Further, the voltage management system should provide an indication of the presence of an invalid or unsupported device and if a newly connected device excessively loads the power supplies.
Accordingly, the present invention is to a measurement test instrument and associated voltage management system for one or more accessory devices, such as a measurement probe, video camera, signal source, device under test or the like. The measurement test instrument includes at least a first accessory device having a memory device coupled to the voltage management system. The voltage management system has at least a first accessory device interface disposed in the measurement test instrument having a sensing line, a voltage input line, clock and data lines, and at least a first voltage power input line wherein the voltage input line provides a voltage to the memory device disposed in the accessory device. A sensing circuit receives a sensing signal from the accessory device via the interface sensing line when an accessory device is coupled to the accessory interface. The sensing circuit generates an interrupt signal in response to the sensing signal that is coupled to a controller. The controller initiates a clock signal to the accessory device via the interface clock line in response to the interrupt signal to retrieve accessory data stored in the memory device of the accessory device via the data line. The controller determines from the retrieved accessory data if the connected accessory device is a valid device and capable of being supported by the measurement test instrument. The controller generates an enable signal for a valid and supported device that is coupled to a voltage switching circuit. The voltage switching circuit generates at least a first output voltage in response to the enable signal that is coupled to the accessory device via the voltage power input line of the accessory device interface.
In the preferred embodiment of the invention, the voltage switching circuit has a plurality of output voltages that are coupled to the accessory device via individual voltage power input lines of the accessory device interface. For a multi-channel measurement test instrument, the voltage management system has a plurality of accessory device interfaces with each interface capable of accepting an accessory device. Each device generates a sensing signal to the sensing circuit for generating an interrupt signal to the controller. The controller initiates a clock signal to each accessory device to retrieve the respective accessory data from each device to determine if the connected accessory device is a valid device and capable of being supported by the measurement test instrument. The controller generates an enable signal for each valid and supported device and couples the signal to respective voltage switching circuits for each device. The accessory data stored in each of the accessory device includes an output voltage current draw value for each of the output voltages which is retrieved by the controller. The controller has means for summing the current draw values for each of the output voltages for each attached accessory device and means for comparing the summed current draw values to maximum output voltage current draw values. The enable signal is supplied to the voltage switching circuit of the last connected accessory device when the summed current draw values are less than the maximum output voltage current draw values. The output voltage or voltages from the voltage switching circuit is removed from the voltage power input line or lines in response to the controller removing the enable signal to the voltage switching circuit when the interrupt signal is removed in response to the sensing signal being removed from the sensing circuit.
The measurement test instrument may be provided with a display device and the controller having means for generating a warning message that is displayed on the display device when an invalid and unsupported accessory device is connected to the device interface. The warning message generating means may also generate a warning message when an accessory device is connected to the measurement test instrument and the summed current draw values are greater than the maximum output voltage current draw value.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with appended claims and attached drawings.